1. Field of the Invention
The present invention relates to antenna arrangements using focal plane filtering to reduce sidelobes and, more particularly, to antenna arrangements including filtering means disposed at a real focal point between two reflectors of the antenna arrangement which passes therethrough the central ray of a beam launched by a feed for smoothing out discontinuities of the image of the feed aperture illumination in the area of the main reflector. The main reflector is then made slightly oversize to intercept the smoothened-out image of the feed aperture illumination along a line which produces a predetermined level of edge intensity.
2. Description of the Prior Art
In radar systems and in terrestrial and satellite communication systems, various techniques have been used to reduce certain sidelobes and in turn the interference therefrom in adjacent links. In receiving systems, undesired sidelobe signals are generally suppressed by receiving the desired signal at a directional antenna and possible interfering signals at a separate omnidirectional antenna. The derived interfering signals are then used to cancel interference in the desired signal using various circuitry configurations. In this regard see, for example, U.S. Pat. Nos. 3,094,695 issued to D. M. Jahn on June 18, 1963 and 3,202,990 issued to P. W. Howells on Aug. 24, 1965. Alternatively, for transmission purposes U.S. Pat. No. 3,704,464 issued to C. J. Drane, Jr. et al on Nov. 28, 1972 discloses a method for maximizing serial directive gain while simultaneously placing nulls in the far-field radiation pattern of an array of N elements which are arbitrarily positioned. The patented method permits specification of directions of up to N-1 independent pattern nulls and/or sidelobes while assertedly providing maximum gain in some prescribed direction. This control is apparently achieved by varying only the amplitude and phase of the element currents in association with a standard gain formula.
U.S. Pat. No. 3,815,140 issued to W. E. Buehler et al on June 4, 1974 relates to a multiple feed arrangement for microwave parabolic antennas which include a parabolic reflector, and a plurality of individual fed illuminators. Each illuminator alone produces a beam of certain dimensions, and by combining the beams through the use of a predetermined configuration of illuminators, including their number and spacing, the physical configuration of the beam, including sidelobes, may be accurately controlled. Furthermore, certain illuminators may be fed by different information sources, thus resulting in a multiple information beam pattern.
It is also known to suppress selected sidelobes in an antenna arrangement comprising a focusing reflector and a feed arrangement by disposing at the feed arrangement or at the focusing reflector at least two sidelobe suppression means comprising either two feeds, two small antennas or sections of the reflective surface of the main reflector. In this regard see, for example, U.S. Pat. application Ser. No. 201,816 filed for E. A. Ohm, now U.S. Pat. Nos. 4,364,052 and 201,822 filed for H. Miedema on Oct. 29, 1980, now U.S. Pat. No. 4,376,940.
A special kind of sidelobes are the grating lobes associated with phased array antenna arrangements. These sidelobes have been reduced to admissible levels by, for example, disposing a filtering means capable of blocking the grating lobes at any real focal point of the antenna arrangement as disclosed in U.S. Pat. No. 4,259,674 issued to C. Dragone et al on Mar. 31, 1981.
The problem remaining in the prior art is to provide a simple technique for illuminating efficiently the aperture of a reflector antenna to provide a predetermined low level of edge intensity at the reflector for reduced sidelobes.